An object is placed $30\,\text{cm}$ in front of a concave mirror of focal length $20\,\text{cm}$. The image distance from the mirror is:

Using the standard sign convention, $u = -30\,\text{cm}$ and $f = -20\,\text{cm}$ for a concave mirror. The mirror formula $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$ gives $\dfrac{1}{v} = \dfrac{1}{-20} - \dfrac{1}{-30} = -\dfrac{1}{60}$, so $v = -60\,\text{cm}$. The negative sign indicates the image is real and on the same side as the object, $60\,\text{cm}$ in front of the mirror.